In U.S. Pat. No. 3,778,614 there is described a method and apparatus for examining an object by means of X- or .gamma.-radiation. A commercial embodiment thereof has a source of X-rays adapted to transmit a beam of radiation through a planar slice to be examined, detector means to detect the transmitted beam after it has passed through the object and means to sequentially translate and rotate the source and detector means about the object during radiographic examination. During the set-up and calibration of this apparatus, the center of rotation of the scanner, upon which the source and detector means are mounted, is determined and a linear encoder is manually adjusted so as to align the center of the linear encoder with the center of rotation of the scanner. This procedure only provides for calibration of the central position of one translational scan for one particular rotational orientation of the scanner and, furthermore, does not compensate for irregularities in either the scanner per se or the calibration mechanism at other angular orientations of the scanner. Accordingly, the aforesaid calibration technique does not provide measurements by which the axial tomographic system can correct for scanner or calibration means eccentricities which inherently occur at other angular orientations of the scanner.